The present invention relates to an active matrix substrate, an electro-optical device, an electronic device, and a method for manufacturing an active matrix substrate.
In various electronic devices such as liquid crystal display devices having a plurality of pixels, organic EL display devices, image sensors, etc., a switching operation of pixels (loads) is performed by using semiconductor devices (e.g. TFTs, etc., hereinafter TFT) formed on glass substrates or quartz substrates. On the substrates, a pixel circuit including a number of TFTs that perform the switching operation of pixels and a drive circuit including a number of TFTs that control driving of each pixel are formed.
In order to improve performance of TFTs formed on glass substrates, etc., various techniques have been examined in which a semiconductor film made of a large crystal grain is formed so as to form a channel forming region of a TFT with a substantially single crystal grain. For example, a technique in which a semiconductor film of a substantially single crystal state is formed by crystallizing a semiconductor film using a micro hole formed on a substrate as a starting point for growing a crystal is disclosed in the following examples: Single Crystal Thin Film Transistor, IBM Technical Disclosure Bulletin, Aug. 1993, pp 257–258; and Advanced Excimer-Laser Crystallization Techniques of Large Grain on Glass. R. Ishihara, et al., Proc. SPIE 2001, vol. 4295, pp 14–23.
By forming a TFT using a semiconductor film of a substantially single crystalline state, a forming region of one TFT (particularly, a channel region) can be constructed with one substantially single crystal grain. This makes it possible to realize a semiconductor device having exceptional electrical characteristics in field effect mobility, etc.
By using such TFT having exceptional electrical characteristics, a drive circuit that operates at high speed with high functions can be formed on a glass substrate or a quartz substrate. However, using a single crystal for a semiconductor film of a TFT causes a problem of a type of body floating effect called parasitic bipolar effect that is seen in silicon on insulator (SOI) devices. The resulting parasitic bipolar effect causes various problems such as dropping a breakdown voltage between source and drain, an abnormally steep sub-threshold characteristic exceeding theoretical limit, increasing an off current, dropping a threshold voltage, etc.
In order to solve the problems, a technique is proposed, for example, in the Japanese Unexamined Patent Publication No. 5-134272. In the technique, a TFT included in a drive circuit is formed using a semiconductor film of a substantially single crystalline state, while a TFT included in a pixel circuit is formed using a semiconductor film of a substantially polycrystalline state.
In the TFT formed using the semiconductor film of the polycrystalline state, the above-mentioned problems seldom occur in commonly used voltage level due to a short life (life time) of minority carrier. This makes it possible to form a drive circuit that can operate at high speed with exceptional electrical characteristics, and a pixel circuit having a low off current.
However, the technique disclosed in the Japanese Unexamined Patent Publication No. 5-134272 has a problem in that a semiconductor film of a single crystal state is formed at high temperature. Specifically, a temperature of 960 degrees centigrade is required to form a single crystal silicon film by reduced pressure epitaxy, while a temperature of 600 degrees centigrade is required to form a single crystal silicon film by solid phase epitaxy. Such manufacturing method required high temperature has a problem in that expensive fused quartz substrates, etc., should be used whereas inexpensive glass substrates or plastic substrates can not be used.